TenTec Rebel & the QRP Skunkwerx Universal Shield

Huntsville Alabama has a great hamfest every August.  In addition to a nice air conditioned indoor boneyard and loads of commercial vendors, Craig, NM4T, puts together a great slate of QRP-related forums.  Not only that, but he also hosts a gathering of QRPers on Saturday night at nearby Monte Sano State Park.  The festivities include an FB BBQ, along with an event or two.  This year, one of the events was the "Huntsville Rebelthon".

Craig, along with Glen, KW5GP, and some others, have formed the QRP SkunkWerx - a design group that, so far as I know, has concentrated primarily on expanding the TenTec Rebel Open Source QRP Transceiver.  The SkunkWerx have come up with a "Universal Shield" for the Rebel, and have pushed the little rig beyond its CW limits by coaxing the rig to work on JT65.

The Rebelthon was limited to a handful of Rebel-owning participants.  Glen provided the Universal Shields, some miscellaneous parts to slightly modify the Rebel, a Nokia 5110 display, the wiring needed to hook it all up, the revised sketch, as well as assistance and general moral support for several of us to start down the road of personalizing our Rebels.

SkunkWerx Universal Shield and Bandswitch Mod
The SkunkWerx mod involved replacing the SPDT on-off switch with a DPDT-center-off switch to enable hardware bandswitching.  The switch sets the band jumper open or short, which tells the chipKIT UNO32 which band the rig is on.  In my case, the bandswitch is wired so that up is 40 meters and down is 20 meters - center is off.

The band changing requires that the DDS tune the appropriate frequency to mix with the IF so the rig will tune on the chosen band.  Also, the appropriate bandpass filter must be selected.  As the rig comes, the bandpass filters are selected with four jumpers.  The SkunkWerx Universal Shield uses two latching relays to select the proper filter.  The RebelAllianceMod does much the same, but with a couple differences.  First, they do not use latching relays - when powered, their relays will constantly consume power - not a lot, but for a QRP rig that often runs on batteries, every little bit helps.  The other difference is that they use a long press of the FUNCTION button on the Rebel to change bands. This has the advantage of not having to re-start the rig when changing bands.  Perhaps someone will incorporate that switching method into the SkunkWerx sketch.

That's one of the beauties of Open Source - many different people contributing different solutions to the same problem - and having these solutions available, others can combine them in ways that appeal to them.

SkunkWerx Nokia 5110 Display
Back to the Universal Shield, the SkunkWerx have included a connector for a display, and there are options for a variety of display types.  Their sketch includes code for the Nokia 5110, as well as a "standard" LCD...which is included in the original TenTec sketch.  Glen provided a Nokia 5110 display, and the information that their sketch provides is shown to the right.  Namely, TX frequency, RX frequency, RX Bandwidth, DDS Step Size, and frequency Band.

At the Rebelthon, Glen made sure everyone had success in modifying their on/off switch into a bandswitch, and also had all the jumpers wired in correctly.  He then uploaded the SkunkWerx sketch into each rig, and tested to make sure all was well.  In my case, the bandswitch was a little flaky, so Glen pulled another pre-wired switch out of his stash and we had my rig flying right in just a couple minutes.

So, with a big push from Glen, it was time to get down to playing more with the Rebel.  As the Rebel comes, it's an OK, but not great, QRP rig.  Certainly adding a display adds a lot to the enjoyment of using the rig.

One of the first things I noticed when I got my Rebel was how bad the RX sounded.  Listening on earbuds was extremely uncomfortable.  There is also a fair bit of noise just from turning the main tuning encoder - or pressing the switches or turning the RIT knob for that matter.  I know that Michael, KD4SGN, had some similar experiences with his Rebel and has made some modifications that have helped.  I need to get with him to learn about what he's done, but in the interim, I decided to connect a speaker to the rig, and the radio is now usable - not the equal of the Drake R-4, but at least now I can stand to listen to it.  In fact, I'm listening to a bit of DX on 40m as I type this.

With the sound of the RX tamed down a bit, I felt better about messing with the sketch.  For starters, I find the Rebel's Wide and Medium bandwidths pretty useless, so I changed the default bandwidth from Wide to Narrow...pretty easy!  Another thing I didn't like about the Rebel is that the finest DDS step is 100 Hz.  I knew 10 Hz steps would make for a more analog-VFO-like experience, and allow me to tune the pitch in just so.  Well, it is easy enough to change the tuning steps, but the Rebel's encoder is so coarse, that the tuning rate is much too slow when using 10 Hz steps.  So, back to 100 Hz for now.

Now, on to the SkunkWerx sketch.  The first thing I didn't really care for with Glen's display was the lack of decimal points and thousands separators on the frequency.  A frequency readout of 14021035 was just too much for me...I could much better deal with 14.021.035 and would prefer 14.021.0 or .04.  Well, one thing I learned was that putting those decimals in isn't the easiest thing to do!  I tried several different methods, but never came up with anything satisfactory.  So, I changed the display to read in KHz, with no thousands separators and no fractional KHz.  Again, not ideal, but something I can re-visit in the future.

Next, the Nokia display is pretty small, and the library Glen used results in six lines, so the type is small as well.  Part of this is because of all the information displayed - Glen uses five of the six available lines. Looking at all the info displayed, really a lot of it is redundant.  I didn't need the Band - that info is available from the frequency readout.  Likewise, the Step Size and Bandwidth are both indicated on the radio's front panel via LEDs, so that could go.  And the main advantage to me for showing both TX and RX frequency is to see how far from zero the RIT is tuned - the RIT doesn't have an "off" position, so it's easy to bump the knob and be a few Hz...or few hundred Hz off frequency.  So, some kind of RIT display would be nice.

AA4GA 5110 Display - as of early September 2014
For my changes, I initially removed everything but the TX frequency from the display.  I then changed to the largest numeric font in the 5110 library, which is effectively three rows high....resulting in a nice, easily visible frequency readout.  Below that, I added a numeric indication of RIT tuning +/- in Hz.  To make it more readable, I skipped a line below the frequency.  That still left a line free at the bottom of the display, and I decided that would be a great spot for an S-meter bar graph.  So, I added that using asterisks for the graphing elements.  I would prefer something more "solid", but that can wait until another day.  I am also displaying the raw signal strength provided by the Rebel so I can get a feel for how high the numbers actually go.  Someday I may use this for "calibrating" the S-meter, but for now, it's nice to just have a relative display of received signal strength.

I do want to mention that my fault-finding of Glen's display isn't a negative reflection on Glen - he actually designed this as a springboard for others to mold into their vision of what the rig should be.  In fact, on Glen's personal Rebel, he's using an OLED display...an option worth considering!  In fact, Glen is not only a capable programmer, he has recently authored a book on using the Arduino for Ham Radio that the ARRL has just published.  I bought the book and have ordered sensors for some of the included projects.  Glen's motivation is definitely educating people on using the Arduino (and by association, the Rebel).

So, if you've got a Rebel, consider using the SkunkWerx Universal Shield and sketch as a start for your own Rebel excursions.  The complete SkunkWerx sketch and documentation of how to build the Universal Shield are available at the Rebel Yahoo Group's file section.

My main purpose in getting the Rebel was to use it for learning about rig control (primarily VFO and display) for use in some future home-brew projects.  I think the rig will come in handy for that and will provide me with a playground to test different ideas - I've already got several more that I want to try when I get around to it.

Thank you to Glen, Craig, and all the SkunkWerx....and to TenTec for the fun toy!